1. Field of the Invention
This invention relates to a method of making ceramic articles by use of a novel composition for the manufacture of ceramics.
2. Description of the Prior Art
Ceramic articles made by firing powder materials such as alumina, ferrite, tungsten carbide, silicon nitride and silicon carbide have come to be used in a wide range of applications including electrical insulating materials, magnetic materials, electronic components, mechanical parts, automobile parts and cutting tools.
Conventionally, such ceramic articles are made by various methods including doctor blade coating, dry pressing, extrusion molding, slip casting and the like. Where a ceramic material such as alumina is used for example in the manufacture of integrated-circuit bases, alumina powder is dispersed in a solution containing an organic binder and a plasticizer to prepare a slurry. Then, using a doctor blade or similar instrument, this slurry is spread in a layer, for example, on a polyester film and dried. The sheet so formed is called a "green sheet". This sheet is rolled up and stored. Depending on need, pieces are cut out of the sheet with a punching die and then fired to decompose the organic binder and the plasticizer and, at the same time, sinter the alumina, thus producing ceramic bases. In this case, cellulosic resins such as ethyl cellulose and butyral resins such as polyvinyl butyral have conventionally been used as the organic binder. However, when these resins are used as such, the resulting green sheet has the disadvantage of exhibiting poor flexibility and, therefore, being easily broken or cracked while it is stored in roll form. Moreover, these resins are also disadvantageous in that their poor thermal decomposability tend to result in the presence of residual carbon in the final ceramic products.
On the other hand, the dry pressing process comprises dispersing ceramic powder in water containing an organic binder, a plasticizer, a dispersant and other agents to prepare a slurry, forming this slurry into granules by means of a spray dryer, and then shaping them by means of a compression molding machine, rubber pressing machine, hot pressing machine or similar machine. Alternatively, the aforesaid slurry can also be shaped by slip casting or extrusion molding. Then, the resulting shaped bodies are fired to produce a variety of ceramic articles.
If the unfired, shaped bodies have low green strength, they are often cracked or chipped during the succeeding binder removal and firing steps. Accordingly, the unfired, shaped bodies must have sufficiently high green strength in order to achieve a reduction in rejection rate and hence an improvement in productivity. On the other hand, in order to prevent the die from being damaged and to produce a dense shaped body containing few pores, it is necessary that the granules used as the starting material be easily collapsed and shaped under moderate pressure during the shaping step. To this end, it is desirable to use an organic binder having well-balanced performance characteristics. An additional property required for organic binders is good thermal decomposability. An organic binder having poor thermal decomposability requires high temperatures and long periods of time in the binder removal step. In some cases, residual carbon may remain in the final ceramic product, resulting in a significant reduction in electrical, magnetic, physical or mechanical properties.
Typical organic binders which have conventionally been used in such processes as dry pressing, extrusion molding and slip casting include water-soluble binders such as polyvinyl alcohol, alkali metal salts of carboxymethyl cellulose and alkali metal salts of acrylic resins. However, these binders have a number of disadvantages. Specifically, polyvinyl alcohol has been commonly used because of its low price, but it may gather mold during storage and may give off harmful gases and a bad odor, thereby bringing about an undesirable working environment. Moreover, the granules formed with polyvinyl alcohol are so hard that considerable damage is caused to the shaping die. In addition, since the granules are difficult to collapse, they produce shaped bodies which contain many pores and tend to become cracked and/or chipped. Furthermore, polyvinyl alcohol exhibits poor thermal decomposability during the firing step, so that the presence of much residual carbon prevents the production of dense ceramic articles. On the other hand, alkali metal salts of carboxymethyl cellulose give high viscosity even when used in low concentrations, and are difficult to handle because of their thixotropic properties. Moreover, like alkali metal salts of acrylic resins, they exhibit poor thermal decomposability. Thus, similar to polyvinyl alcohol they also fail to produce dense ceramic articles.